1. Field
The present invention relates generally to fifth wheel assemblies, and in particular, to fifth wheel assemblies with direct-mount transverse beams and compatibly configured fifth wheels.
2. Description of the Related Art
Fifth wheel hitches for towing a trailer behind a towing vehicle have long been known in the art. Examples of fifth wheel hitches include those disclosed in U.S. Pat. Nos. 4,721,323, 5,449,191, and 5,707,070.
With reference to FIGS. 1A and 1B, large over-the-road freight trailers 119 are usually coupled to tractor trucks 109 by means of a fifth wheel coupling 101 whereby the truck rear drive axles directly support a portion of the trailer 119 load burden. Usually, the fifth wheel coupling 101 and support point is located along the length of the tractor 109 between the rear drive wheels and the front steering wheels thereby distributing the front trailer 119 load.
Typically, a fifth wheel coupling 101 includes a support frame for mounting the coupling 101 to the towing vehicle 109, such as, for example, to a tractor truck. This support frame includes a pair of base rails that are usually bolted to the bed and/or frame of the truck and that are tied in parallel by tie bars. Side brackets are mounted to the base rails, either directly or through a pedestal which may be longitudinally adjustable. A fifth wheel 130 is mounted to the side brackets by means of a trunnion arrangement allowing for pivotal movement in the vertical plane. The fifth wheel 130 comprises a top bearing surface, or plate, and a slot opening toward the aft end of the towing vehicle for receiving a trailer kingpin, the slot bounded laterally by a pair of ramped tips. Fifth wheel 130 is an assembly which includes a jaw assembly, or locking mechanism that is specifically adapted to releasably engage and hold a kingpin 115 of a trailer desired to be towed by the towing vehicle 109. Often, base rails are mounted to angles which are then mounted to the truck frame.
Prior art fifth wheels are typically formed of steel or ductile iron and incorporate an integral load structure comprising a set of walls that serve to transfer the loads experienced by the bearing plate to the support and then to the tractor, and to house the kingpin locking mechanism. The load structure further comprises pivot bearings which rest upon the side brackets and permit pivotal movement of the bearing plate 130 in the vertical plane.
Heretofore, fifth wheels have been manufactured such that the bearing plate and load structure are integrated, either through casting to produce a unitary fifth wheel, or by welding the two components together. Thus, traditional fifth wheel mounting systems are designed such that the fifth wheel accounts for a majority of the stiffness and load carrying ability. The load path on traditionally mounted fifth wheels depends upon each of the mounting brackets being positioned directly over the truck frame rails. This minimizes the bending stress in the base rails and tie bars. As a result of this requirement, the width of traditional fifth wheels has been controlled by the width of the truck frames. Such fifth wheel assemblies of the prior art are bulky, requiring a significant amount of material to manufacture. They also add to the weight of the tractor.